Here you can download sheet music from all the different Fire Emblem games! Sheet music are sheets of paper that have the tracks and notes that make up a song written on them. MIDI's are sheet music in digital form that can be played back through the computer.

MIDI's and sheet music are interchangeable however MP3's and MIDI's are NOT interchangeable. MP3's are recordings of sounds while MIDI's are files
that simulate sheet music (essentially a list of notes to play) in digital form so that the music can be heard.
The two file types are too different to be interchanged properly.

Below is the largest archive of Fire Emblem sheet music and MIDI's. Programs that can convert MIDI's to sheet music and sheet music to MIDI's can be found here.